A vessel carrying Guyana's first-ever shipment of crude set sail on Monday bound for the U.S. Gulf Coast, oil major Exxon Mobil Corp said, marking the tiny South American nation's long-awaited debut as an oil exporter.

The Suezmax vessel Yannis P, loaded with about 1 million barrels of Liza light sweet crude was bound for Galveston, Texas, according to tracking service TankerTrackers.com, and departed from the floating platform Liza Destiny off Guyana. 

Exxon and partners Hess Corp and China's CNOOC started production at Guyana's promising Stabroek block in December, ahead of schedule, after finding more than 6 billion barrels of recoverable oil and gas. Exxon had said on Friday that the project's first crude shipment would load during the weekend and head for its refining network. 

The U.S. company operates refineries in Louisiana and Texas in addition to plants inland in Montana and Illinois. All commercial oil resources in Guyana, an impoverished nation whose economy is expected to be transformed by its emerging oil industry, have been found at the Stabroek offshore block, where Exxon and its partners have disclosed 15 large discoveries so far. 

The Exxon-led venture alone expects to produce at least 750,000 barrels per day of crude by 2025, enough for Guyana to become an oil power alongside neighboring Venezuela and Brazil. Guyana's transition to an oil-producing country has sparked a political controversy in the nation's ongoing presidential campaign over the way the government gave exploration and production rights and designed its royalty framework. 

Guyana's People's Progressive Party (PPP) has criticized President David Granger's 2016 deal with Exxon as too generous and pledged to "immediately engage the oil and gas companies in better contract administration/re-negotiation." 

Exxon said the negotiated terms of the contracts "are competitive with other agreements signed in countries at a similar resource development phase." Exxon is expected to export both its first two cargoes of Guyanese oil. 

Another of the first shipments from the Liza field is scheduled to be taken by U.S.-based Hess in February, with others earmarked for sale by the Guyanese government through an open-market tender won by Royal Dutch Shell. (Reporting by Marianna Parraga and Neil Marks in Georgetown; Editing by Tom Brown and Daniel Wallis)